1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device and a method of manufacture thereof.
2. Description of the Related Art
Memory cells used in a programmable read only memory (PROM), one type of non-volatile semiconductor memory device, are each formed of a non-volatile transistor for storing data and a transistor for selecting the memory cell.
FIG. 1 illustrates a sectional view of a non-volatile transistor used in memory cells of a conventional PROM. A field oxide layer 12 is formed over the surface of a P-type silicon substrate 11. An N.sup.+ -type source region 13 and an N.sup.+ -type drain region 14 are formed in the substrate to be surrounded by a field oxide layer 12. A first gate insulation layer 15, a floating gate electrode 16 of polysilicon, a second gate insulation layer 17 and a control gate electrode 18 of polysilicon are sequentially formed over a portion of the substrate between source region 13 and drain region 14.
With such a transistor as described above, high voltages are applied to gate electrode 18 and drain region 14 so that hot electrons are generated in the vicinity of the drain within a channel region between the source and drain regions, and the hot electrons are injected into floating gate electrode 16 and stored therein so that data is written into the memory cell.
To construct the PROM, a great number of non-volatile transistors as described above must be formed on the substrate. And, to realize a large-capacity memory, the dimensions of the non-volatile transistor in the longitudinal and horizontal directions thereof should be made very small. However, in conventional PROMs using non-volatile transistors having such a structure as shown in FIG. 1, simply decreasing the dimensions of each non-volatile transistor on a plane would degrade reliability and separation characteristics of transistors.